fix error handling during querying result rows

[dhlin]

When issuing a long running request with the client, if the query get killed, conn.Execute will be hanging.

How to reproduce:

1. issue such request below

conn, _ := client.Connect("127.0.0.1:3306", "root", "", "db")
conn.Ping()
r, err := conn.Execute(`select sleep(1000), id from table`)
if err != nil {
    fmt.Println(err)
    return
}

2. use mysql cli to find the query id with show processlist; and kill the query with kill query id;

the program will hang on conn.Execute and never return.